The present invention relates to an emission treatment system having an oxidation catalyst upstream of a soot filter coated with a material effective in the Selective Catalytic Reduction (SCR) of NOx by a reductant, e.g., ammonia. In one embodiment, the system provides an effective method of simultaneously remediating the nitrogen oxides (NOx), particulate matter, and gaseous hydrocarbons present in diesel engine exhaust streams.
Diesel engine exhaust is a heterogeneous mixture which contains not only gaseous emissions such as carbon monoxide (“CO”), unburned hydrocarbons (“HC”) and nitrogen oxides (“NO,”), but also condensed phase materials (liquids and solids) which constitute the so-called particulates or particulate matter. Often, catalyst compositions and substrates on which the compositions are disposed are provided in diesel engine exhaust systems to convert certain or all of these exhaust components to innocuous components. For example, diesel exhaust systems can contain one or more of a diesel oxidation catalyst, a soot filter and a catalyst for the reduction of NOx.
Oxidation catalysts that contain platinum group metals, base metals and combinations thereof are known to facilitate the treatment of diesel engine exhaust by promoting the conversion of both HC and CO gaseous pollutants and some proportion of the particulate matter through oxidation of these pollutants to carbon dioxide and water. Such catalysts have generally been contained in units called diesel oxidation catalysts (DOC's), which are placed in the exhaust of diesel engines to treat the exhaust before it vents to the atmosphere. In addition to the conversions of gaseous HC, CO and particulate matter, oxidation catalysts that contain platinum group metals (which are typically dispersed on a refractory oxide support) also promote the oxidation of nitric oxide (NO) to NO2.
The total particulate matter emissions of diesel exhaust are comprised of three main components. One component is the solid, dry, solid carbonaceous fraction or soot fraction. This dry carbonaceous matter contributes to the visible soot emissions commonly associated with diesel exhaust. A second component of the particulate matter is the soluble organic fraction (“SOF”). The soluble organic fraction is sometimes referred to as the volatile organic fraction (“VOF”), which terminology will be used herein. The VOF can exist in diesel exhaust either as a vapor or as an aerosol (fine droplets of liquid condensate) depending on the temperature of the diesel exhaust. It is generally present as condensed liquids at the standard particulate collection temperature of 52° C. in diluted exhaust, as prescribed by a standard measurement test, such as the U.S. Heavy Duty Transient Federal Test Procedure. These liquids arise from two sources: (1) lubricating oil swept from the cylinder walls of the engine each time the pistons go up and down; and (2) unburned or partially burned diesel fuel.
The third component of the particulate matter is the so-called sulfate fraction. The sulfate fraction is formed from small quantities of sulfur components present in the diesel fuel. Small proportions of SO3 are formed during combustion of the diesel, which in turn combines rapidly with water in the exhaust to form sulfuric acid. The sulfuric acid collects as a condensed phase with the particulates as an aerosol, or is adsorbed onto the other particulate components, and thereby adds to the mass of TPM.
One key aftertreatment technology in use for high particulate matter reduction is the diesel particulate filter. There are many known filter structures that are effective in removing particulate matter from diesel exhaust, such as honeycomb wall flow filters, wound or packed fiber filters, open cell foams, sintered metal filters, etc. However, ceramic wall flow filters, described below, receive the most attention. These filters are capable of removing over 90% of the particulate material from diesel exhaust. The filter is a physical structure for removing particles from exhaust, and the accumulating particles will increase the back pressure from the filter on the engine. Thus, the accumulating particles have to be continuously or periodically burned out of the filter to maintain an acceptable back pressure. Unfortunately, the carbon soot particles require temperatures in excess of 500° C. to burn under oxygen rich (lean) exhaust conditions. This temperature is higher than what is typically present in diesel exhaust.
Provisions are generally introduced to lower the soot burning temperature in order to provide for passive regeneration of the filter. The presence of a catalyst promotes soot combustion, thereby regenerating the filters at temperatures accessible within the diesel engine's exhaust under realistic duty cycles. In this way a catalyzed soot filter (CSF) or catalyzed diesel particulate filter (CDPF) is effective in providing for >80% particulate matter reduction along with passive burning of the accumulating soot, and thereby promoting filter regeneration.
Future emissions standards adopted throughout the world will also address NOx reductions from diesel exhaust. A proven NOx abatement technology applied to stationary sources with lean exhaust conditions is Selective Catalytic Reduction (SCR). In this process, NOx is reduced with ammonia (NH3) to nitrogen (N2) over a catalyst typically composed of base metals. The technology is capable of NOx reduction greater than 90%, and thus it represents one of the best approaches for achieving aggressive NOx reduction goals. SCR is under development for mobile applications, with urea (typically present in an aqueous solution) as the source of ammonia. SCR provides efficient conversions of NOx as long as the exhaust temperature is within the active temperature range of the catalyst.
While separate substrates each containing catalysts to address discrete components of the exhaust can be provided in an exhaust system, use of fewer substrates is desirable to reduce the overall size of the system, to ease the assembly of the system, and to reduce the overall cost of the system. One approach to achieve this goal is to coat the soot filter with a catalyst composition effective for the conversion of NOx to innocuous components. With this approach, the catalyzed soot filter assumes two catalyst functions: removal of the particulate component of the exhaust stream and conversion of the NOx component of the exhaust stream to N2.
Coated soot filters that can achieve NOx reduction goals require a sufficient loading of SCR catalyst composition on the soot filter. The gradual loss of the catalytic effectiveness of the compositions that occurs over time through exposure to certain deleterious components of the exhaust stream augments the need for higher catalyst loadings of the SCR catalyst composition. However, preparation of coated wall flow soot filters with higher catalyst loadings can lead to unacceptably high back pressure within the exhaust system. Coating techniques that allow higher catalyst loadings on the wall flow filter, yet still allow the filter to maintain flow characteristics that achieve acceptable back pressures are therefore desirable.
An additional aspect for consideration in coating the wall flow filter is the selection of the appropriate SCR catalyst composition. First, the catalyst composition must be durable so that it maintains its SCR catalytic activity even after prolonged exposure to higher temperatures that are characteristic of filter regeneration. For example, combustion of the soot fraction of the particulate matter often leads to temperatures above 700° C. Such temperatures render many commonly used SCR catalyst compositions such as mixed oxides of vanadium and titanium less catalytically effective. Second, the SCR catalyst compositions preferably have a wide enough operating temperature range so that they can accommodate the variable temperature ranges over which the vehicle operates. Temperatures below 300° C. are typically encountered, for example, at conditions of low load, or at startup. The SCR catalyst compositions are preferably capable of catalyzing the reduction of the NOx component of the exhaust to achieve NOx reduction goals, even at lower exhaust temperatures.
The prior art contains descriptions of the use of SCR catalyst compositions, soot filters and combinations thereof for the abatement of both the NOx and particulate components of diesel exhaust. These references are described below.
Japanese Kokai 3-130522, for example, discloses the treatment of diesel exhaust gases characterized by use of an ammonia injector and porous ceramic filter having a denitration catalyst within the pores. The filter is installed in the wake of the diesel engine exhaust. The ceramic porous filter comprises an upstream fine pore path layer, and a downstream side course ceramic particle layer on which the denitration catalyst was supported. The fine layer can support a platinum or palladium or other hydrocarbon combustion catalyst. The diesel exhaust gas containing unburned carbon flows through the porous ceramic filter and the carbon particles are filtered onto the surface. The gas containing nitric oxides and the ammonia passes through the denitration catalyst containing side of the filter and the nitric oxides are reduced to nitrogen and water. The oxidation catalyst on the upstream side causes the particulate component to burn off catalytically.
U.S. Pat. No. 4,912,776 discloses an oxidation catalyst, an SCR catalyst downstream and adjacent to the SCR catalyst, and a reductant source introduced to the exhaust stream between the oxidation catalyst and the SCR catalyst. Providing a higher feed containing a high proportion of NO2 to NO to the SCR reactor is said to allow the use of lower temperatures and higher space velocities than is possible with a feed of NO.
WO 99/39809 discloses a system for treating combustion exhaust gas containing NOx and particulates that has an oxidation catalyst effective to convert at least a portion of the NO in the NOx to NO2, a particulate trap, a source of reductant fluid and an SCR catalyst. The particulate trap is downstream of the oxidation catalyst; the reductant fluid source is downstream of the particulate trap; and the SCR catalyst is downstream of the reductant fluid source. Reductant fluids disclosed include ammonia, urea, ammonium carbamate and hydrocarbons (e.g., diesel fuel).
A catalytic wall flow filter for an exhaust system of a combustion engine is described in WO 01/12320. The wall flow filter has channels that are in honeycomb arrangement, where some of the channels are blocked at the upstream end and some of the channels that are unblocked at the upstream end are blocked at the downstream end. An oxidation catalyst is disposed on a gas impermeable zone at an upstream end of channels that are blocked at the downstream end. The filter has a gas permeable filter zone that is downstream of the oxidation catalyst that is for trapping soot. The oxidation catalyst is described to be capable (when in an exhaust system) of generating NO2 from NO to combust the trapped soot continuously at temperatures below 400° C. The oxidation catalyst preferably includes a platinum group metal. Exhaust streams containing NO are initially passed over the oxidation catalyst to convert NO to NO2 prior to filtering to remove soot. The exhaust gas then containing NO2 is used to combust the soot trapped on the filter.
In some embodiments of the wall flow filter described in WO 01/12320 the downstream channels of the soot filter contain a catalyst for a NOx absorber and an SCR catalyst downstream of the NOx absorber. The SCR catalyst can be a copper-based material, platinum, a mixed oxide of vanadium and titania or a zeolite, or mixtures of two or more thereof.